a) Field of the Invention
The invention is directed to an arrangement and a method for polarization-optical interference contrast.
b) Description of the Related Art
It is applicable in particular to high-contrast microscope display of phase objects such as cell cultures arranged on any (optically isotropic or anisotropic) substrate materials by means of differential interference contrast (DIC).
Objects to be examined by microscope can be distinguished as amplitude objects and phase objects. While amplitude objects change the brightness or color of the light used for display and can therefore be made directly visible, phase objects only change the phase position of the light, e.g., by diverging from the surroundings with respect to refractive index or thickness. Since neither the human eye nor electronic image converters can detect changes in phase position, additional steps must be taken to display phase objects in order to translate phase changes into amplitude changes.
Different methods distinguished as phase contrast methods and interference contrast methods have been developed for this purpose.
One of the first interference contrast methods was proposed in 1947 by Francis Smith in British Patent GB 639 014. For this purpose, a Wollaston prism is introduced into the focal plane of the condenser and into the focal plane of the objective in the beam path of the microscope. A polarizer is arranged in the beam path in front of the first Wollaston prism for generating linearly polarized light. After passing through the second Wollaston prism, the light is analyzed (analyzer) by means of a second polarizer. The illumination-side Wollaston prism has the task of compensating the pupil path difference generated by the Wollaston prism on the imaging side, making it possible to work with the full illumination aperture.
This method was further developed in 1952 by Georges Nomarski in French Patent FR 1 059 123. In this case, the spatial orientation of the Wollaston prisms is modified so that splitting and recombination of the ordinary and extraordinary beam can be relocated to a plane outside of the prism, so that a better possibility for the arrangement of the optical elements is achieved.
In 1963, Maximilian Pluta proposed (U.S. Pat. No. 3,495,890) replacing the Wollaston prism on the imaging side by two Wollaston prisms arranged one behind the other, so that the image splitting size could be changed discretely in three steps.
The DIC microscope recently suggested by Kenichi Kusaka in U.S. Pat. No. 6,229,644 also has an analogous construction.
All of these solutions have in common that the illumination of the object must be carried out with polarized light. Accordingly, the objective as well as all optical elements in the illumination beam path must have an isotropic character. Optical anisotropism in these elements would substantially interfere with the image character of the intermediate image. In particular, no object carriers made of plastic can be used because this would have a birefringent effect. This rules out the plastic petri dishes used as object carriers for cell cultures. For purposes of analysis, the cell cultures must be arranged on conventional cover slips, which requires additional preparation time and costs and precludes examination of live cultures.
In order to eliminate these disadvantages, it is suggested in DE-OS 100 04 135 to use special petri dishes with defined optical characteristics. However, the manufacture of these petri dishes and selection of usable materials are costly.